Coordinated optical, ultraviolet, radio and X-ray observations of supernova 1979c in M 100

Optical, ultraviolet, radio and X-ray observations of supernova 1979c in M 100 made in the first six weeks after discovery (1979 April 20 - June 5) are presented and discussed. On the basis of the optical data, the SN is classified as peculiar type II. It attained a magnitude at maximum of Msub(B) = - 19.4 +- 0.8 mag and has decayed linearly at a rate of 0.046 mag day-1. Correspondingly, the colour temperature decreased from 1.1 x 104 K on April 22 to approximately 7 x 103 K in late May. The total radiative energy is estimated to be Esub(rad) = 7 x 1049 erg. The optical lines, as well as the Mg II 2800 A doublet, originate in the upper layers of the main SN envelope, which is moderately ionized and expands at about 104 km s-1. The main results from the ultraviolet line spectrum are that: (i) Most absorption features are produced either in the disc or the halo of our Galaxy and M 100. (ii) The emission lines indicate the presence of a shell where the gas is highly ionized, the expansion velocity is around 4 x 103 km s-1 and the radius is greater than twice the photospheric radius. This shell is likely to consist of compressed gas of pre-existing circumstellar material. Radio and X-ray observations have provided only upper limits to the SN emission. The inferences from these limits confirm the ultraviolet results. (author)